Are we related? I refuse to as well. I have gone out of my way to help others in the past, at totally random times and places. Consequences for myself be damned. I refuse not to!
I would have asked the kid a few more questions, then gone over there and said to the mum look, I just saw your son walking to the shop..can I help? Can I at least go to the shop for you? I would not have been judgmental with anything I may have found, and I would have treated them with respect and dignity. She could be too proud to get help, or have no family or friends to rely on, and could be at the end of her tether. The kid shouldn't be walking that far to the shops on his own either. For his own personal security and his own well being. He is 5 yrs old! Poor little man.
I get that males are too scared to approach kids in public, which is sort of understandable given that so many are quick to judge. In that case, find a female to approach with help.
Whatever it took. What I wouldn't do is ignore it. Sometimes a bit of kindness and compassion is all it takes for others.
There is no can't, there is only try.